Supervisory means for communicating channels



Patented Dec. 10, 1940 PATENT OFFICE SUPERVISORY MEANS FOR COMMUNI- C'ATING CHANNELS Simeon C; Leyland, Bloomfield, N. J., assignor to Westinghousev Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application November 4, 1939,v Serial No.v 302,908

zo claims. (c1. ,1W-311)" y My invention relates to the supervision of communicating channels, and particularly the supervision of a carrier-current channel such as is utilized for protecting transmission lines 5 against faults, and for inter-station communication over the transmission line or system. My invention relates to the periodical checking of the condition of the carrier-current transmitters and receivers at each end of a protected line-section of a transmission line, particularly Where carrier-current relaying is utilized.

An object of my invention is to provide automatic means, instigated automatically or by an operator, at only one end of the protected line- 1;, section, for obtaining a complete check-back, indicating whether the carrier-current transmittingY and receiving equipments at both ends of the line-section are functioning properly and whether the line section is intact, and if not, indicating which equipment is at fault.Y

A still further object of my invention is toprovide an automatic periodic check-back system for such carrier-current equipments, operative automatically at periodic intervals, and .sounding an alarm-signal to indicate when the equipment at either one or both ends vof the protected linesection is not operating properly.

With the foregoing and other objects in view, my invention consists in the apparatus, parts, combinations, methods and systems hereinafter described and claimed, and illustrated in thearccompanying drawing wherein:l

Figure 1 is a diagrammatic view of circuits and apparatus illustrating my invention in a preierred form of embodiment; v

Fig. 2 is an across the line diagram of the supervisory equipment at a i'lrst'station, M, of the line;

Fig.; 3 is an across the line diagram of the supervisory equipment at asecond station, N, of the line; and Fig. 4is an across the line diagram of a modied form of embodiment of the supervisory equipment for the rst station, M. v

As shown in Fig. 1, my invention is illustrated as being applied to a carrier-current channel which is superimposed upon one phase-conductor of a S-phase transmission line 5 which is illustrated as extending between two diiierentstations designated M and N. The carrier-current channel is provided by means ofl a normally nontransmitting carrier-current transmitter 6 and a carrier-current receiver 1 at a iirst station, M, and a normally non-transmitting carrier-current transmitter 8 and a carrier-current receiver 9 at a second station, N, all of saidl carrier-current apparatus being tuned t-o the same carrier-current frequency. kThe transmitter 6 and receiver 1 at the station M are coupled to the third-phaseconductor of the line 5, through a coupling transformer II and a coupling capacitor I2 at the station M, while thetransmitter 8 and receiver 9 at the station N are coupled to said third phase-conductor of the line 5 through a coupling transformer I3 and a coupling capacitor I4. Suitable Wave-traps' I5r and I6 are utilized for more or less conning the carrier currents to the portion of the line 5 extending between the two stations M and N.

The carrier-current equipment thus far described constitutes a communicating channel such as is utilized in protecting transmission lines against faults, aril example of such a protective system being, shown in the Harder Patent No. 2,144,493, granted January 17, 1939, and assigned to the Westinghouse Electric &.Manufacturing Company. j

While my invention, in its broadest aspects, is not limited to this particular kind of a communication channel, the invention was nevertheless particularly designed. for such a carrier-current protective system, where the. carrier-current channel is utilized primarilyy for protective purposes, to afford protection in theevent of a fault, but usually also the communication channelis, utilized,A at times when there are no faults, for general communication-purposes between the operators at the two stations at the respectivelends of the protected line-section, absolute control of the carrier-,current channel beinggiven to the protective equipment, in the rare momentsvwhen i there is a fault'on the transmission system, regardless of any other uses of the carrier-current channel.

My presentr invention relates to the supervision of such a carrier-current channeLto determine, at more or less frequent intervals, whether the carrier-current equipment is in proper condition for protecting the, ltransmission line against faults, or for the normalpurpose'for which the r'carrier-current channel' isutilized.

' According to my invention, supervisory equipment is provided for causing the transmitter 6 at the first station, M, to begin to transmit carrier-current. This may be done in any one of a number of Ways, the illustrated method of control being to connect the grid I'I oi an oscillator- 'tube I8 to the cathode I9 of said tube, said cathode I9 being also connected, for grid-control purposes, to the positive terminal (Jr) of the stationi battery, the negative terminal of which is indicated at The receiver I at the first station, M, is properly tuned to the carrier-current frequency, and is provided with an amplifier-tube or output-tube 2I having a plate-circuit 22 which energizes a receiver relay RM from a B-battery which is represented by the terminals (B+) and (B-).

At the second station, N, the transmitter 8 is also caused to transmit by connecting its oscillater-grid I'I to its oscillator-cathode I9, the latvter being also connected, for gridecontrol purposes, to the positive terminal of thestation-battery. The receiver 9 lat said' second station, N, is utilized to energize a receiver relay RN which is energized from the BT-battery at this station.

The supervisory equipment at the rst station, M, includes a long-interval timer TK anda short-interval timer TM, which are, in reality, suitably geared synchronous motors energized from a Gil-cycle source 24. At the second station, N, a short-interval timer TN is utilized, similarly energized from a (iO-cycle source at that station. The 60-cycle sources 24 and 25 may, or may not, be tied in with the protected transmission-line 5. Each of the three timers TK, TM and TN is provided with an arm 26, which carries a plurality of moving contact-elements such as 2l, said arm being normally biased, against' a back-stop 2S, by means of a returning-spring 29. When the timer is energized from its {iO-cycle source, its contact-arm 26 moves slowly away from its back-stop 28, at a speed which is determined by the speed of the timer-rnotor and the gear ratio which is utilized. When the timer is disconnected from its 60-cycle source, the. construction is such that theV returnspring 29 quickly returns the movable arm 2B to its initial position against the back stop 28, this return-motion being eiected in any convenient period of time, whichwill be assumed to be 0.1 second, for purposes ofV explaining the invention,

, although any other` convenient resetting-time 5 may be utilized.

The long-interval timer TK has two contacts TK5I] and TKSU', both of which close, in 50 minutes after the timer is energized, or after any other convenient time-interval.

The short-interval timer TM at the stationM has four contacts as follows: contacts TMO, which close instantly, or within a short time after the timer isenergized, and which open near the close of .the check-back` operation, which may be in, say, 6 seconds after the timer is energized; normally closed contacts TM4, which open in 4 seconds after the timer is energized; normally open contacts TM4 which close in 4 seconds after the timer is energized, and which thereafter remain closed as long as the timer is energized; and normally closed contacts TM5 which open in 5 seconds after the timer is energized.

At the second station, N, the short-interval timer IN has three contacts, as follows: normally open contacts TN3 and TN3, both of which close in 3 seconds and remain closed during the rest of the energization of the timer; and normally closed contacts TNI, which open inl 7 seconds after the energization of the timer, and which are utilized to nally terminate the supervisory or check-back operation, as will be subsequently'described.

The relative orders of the short-term timesettings of the timers TM and TN are significant, although their absolute values are not, eX- cept that the shortest interval, here indicated as 3 seconds, should be longer than the longest carrier-current operation which is required in order to properly control the protective relaying equipment (not shown), or other normal-use equipment of the carrier-current channel, it being understood that my present invention relates particularly to the supervisory or checking equipment, for assuring that the channel is in constant readiness for its normal use, regardless oiiwhat that normal use may be.

The receiver. relays RM and RN are of the instantaneous type. The relay RM has three normally open or make contacts RMI, RMZ and RM3, which close when the relay is properly energized. The relay RN has two make contactsRNI and RN2.

In addition to the foregoing equipment, the illustrated form of embodiment of my supervisory apparatus embodies a push-button PBM at the rststation, M, three bells or other signalling-means I, 2 and 3, at the rst station, M, and an auxiliary relay A and a resistor AR, at the `first station, M, the relay A being of the instantaneous typeV and` having two make contactsv AI and A2. At the second station, N, the equipment further includes a push button PBN anda bell .or other signalling-means 4.

The electrical connections of the equipment, and the cycle of operations, are as follows.

The beginning of the long time-interval of 50 minutes, more or less, between successive automatic checking-operations of the supervisory equipment, is initiated by the deenergization of the short-interval timer TM, which re'sults in thek reclosure of its contacts TM5. These contacts, when closed, connect the long-interval timer TK across the 60-cycle mains 24 at the station M. Fifty minutes later, the timer-contacts TK50 and TK50 close. The contact TKSU is connected, in series with the normally closed contact TM4between the oscillator-grid II and the positive terminal (-1-) of the station-battery, and thence also to theoscillator-cathode I9. The oscillator-tube I8 of the carrier-current transmitter 6 may be regarded as being of a type which does not oscillate when its grid I'I is disconnected from its cathode I9, or when it has a negative potential with respect to its cathode I9, but the tube oscillates when the grid I'I has substantially the same potential as the cathode I9. The closure of the timer-contact TK50 thus initiates a supervisory transmission of carrier-current, from the rst station, M.

The second timer-contact TK59 is connected in series with the short-interval timer TM, at the same station M, so as to initiate the operation of said short-term timer, which immediately, or soon, results in the closure of its first timer-contact TMO, which by-passes the timercontact TK50, thus sealing in the short-interval timer TM until it has run its course, as determined by the opening of its sealing-in contact TMO, 5 seconds later.

The initiation of the operation of the carriercurrent transmitter at the rst station, M, results in the instantaneous response of both the receiver 'I at. the rst station, M, and the receiver 9 at the second station, N, thus instantly energizing both of the receiver relays RM and RN, provided that both receivers I and 9 are in proper operating condition, and further provides that the transmitter 6 at the first station, M, is also in proper operating condition. The energization of the two receiver relays RM and RN yresults in the immediate closure of the three relay-contacts RMI, RM2 and RM3 at the station M, and the two relay-contacts RNI and RNZ at the station N.

At the station M, the receiver contact RMI immediately short-circuits the operating coil of the auxiliary relay A, so that it will thereafter be impossible to elfectively energize this auxiliary relay as long as the receiver relay RM remains properly energized. The receiver-relay contact RM2 is connected in series with the bell 2, and also in series with the normally open timer-contact 1M4, so that the closure of the receiverrelaycontact RM2 does not immediately result in the sounding of the bell 2. The receiver-relay contact RMS instantly energizes the bell 3 across the station-battery which is represented by the positive and negative terminals and During an automatic supervisory checkback operation, if the carrier-current transmitter 6 and receiver 'I at the first station, M, are both operating properly, this bell 3 instantly sounds, at the beginning of each supervisory check-back, and it normally remains sounding for about 7.1 seconds, as will subsequently be described.

At'vthe second station, N, the instantaneous closure of the receiver-relay contact RNI results in the connection of the short-interval timer TN across the G50-cycle bus 25 at that station, the timer 'I'N being also connected in series with its normally closed timer-contact TN1. The second Contact, RN2, of the instantaneous receiver relay RN, operates to ring a bell 4 which is illustrated as being energized from the station-battery at the second station, N, as represented by the positive and negative terminals (-1-) and The bell 4, at the second station, N, thus also begins to ring instantly, at the beginning of each automatic supervisory test of the carrier-current equipment, and it also continues to ring for 7.1 seconds, if the equipment is in proper operating order.

The supervisory operations thus far described have all taken place substantially instantaneously upon the closure of the two long-interval timer-contacts TKl and TK. The long-interval timer-contact TKSB, in series with the short-interval timer contact TMA at the first station, M, jointly operates to connect the transmitter-grid II to the transmitter cathode i9, maintaining said carrier-current transmission as long as the grid I.'I remains connected to the cathode I9. This connection is maintained for 4 seconds, or until the short-interval timer-contact TMI opens, which thus causes a discontinuance of the transmission of carrier at the first station, M, 4 seconds after the carrier Was startedy at that station.

Before this 4-'second time-interval has elapsed, the timer TN, at the other station N, will have closed its contacts TN3 and TN3, which close in, say, 3 seconds. The timer-contact TNS, at the station N, connects the transmitter-grid l1 to the transmitter-cathode I9, at that station, thus initiating the operation of the carrier-current transmitter 8 at said station N. The timer-contact TNS', at said station N, by-passes the receiver 9 at that station, and seals inthe receiver relay RN, thereafter maintaining the energization of said receiver relay across the B-battery terminals (B+) and (B This sealing-in of the receiver relay RN assures the maintenance of the receiver-relay contact RNI until the timer 'IN has run its full course, as determined by the timer-contact TN'I which opens in 7 seconds, thus finally deenerglzing the timer TN and returning its timer-contacts to their initial or normal positions. f l A l At the termination of the 4-second interval, the timer TM, at the first station, M,'simu1taneously opens its contact 'FM4 and closes its timer-contact TM4. As previously described, the opening of the timer-contact TM4 interrupts carrier-current 'transmission at that station M. At the same time, or immediately thereafter, the closure of the timer-contact TMII results in a completion of the energization of the bell 2, in series vviththev receiver-relay contact RM2, and across the station-'battery terminals (-1-) and provided, of course, that the transmitter 8 at the other station, N, and the receiver 'I at this station, M, are both operating properly. The ringing of the bell 2, four seconds after the beginning of the ringing of the bell 3, thus signifies that the check-back is complete, that is, that the carrier-current transmitter B and receiver 'I at the first station, M, are both operating properly, and that the carrier-current receiver 9 and transmitter 8 at the second station, N, are also operating properly. If everything is operating properly, this check-back bell 2 rings for, say, 2.1 seconds, or until immediately after the deenergization of the timer TM as a result of the opening of the timer-contact TMO, as Will be subsequently de scribed.

The timer-contact 'IM4', which closes at the end ofthe 4-sec0ndv interval, also completes a circuit through the short-circuited coil of the auxiliary relay A and the resistor AR, across the battery-terminals and the coil of the auxiliary vrelay A being short-circuited by the receiver-relay'contact RMI if the receiver I at the station M is properly receiving carrier current which is transmitted from the transmitter 8 at the station N, at that time.

If the proper carrier-current signal is not received, at the close of this 4-seoond time-interval, the receiver-relay contact RMI Will not be closed, so that the closure of the timer-contact TMll will instantlyenergize the auxiliary relay A, causingv the latter to, permanently seal itself in, through its relay-contact AI, and also to permanently connect the alarm-bell I across the station-battery, through its relay-contact A2, thus indicating that carrier currentis not being received from theA remote station N at the end of this 4-second time interval, at which time it should bereceived, vif everything were in proper order. The Vsilence of the bell 2, as a result of the Vopen condition of. the receiver-relay contact RN2, will also bean indication that the carriercurrent equipment is not operating properly. Meanwhile,"if` the bell 3Vhas been ringing for the first 4 seconds, while the transmitter 6 at this station, M, was transmitting, said bell 3v Will have indicated that the `carrier-current equipment at this station, M, is operating correctly, so that the troublemust beat the other station, N.

, At the' end of the 5-second period after the initiation of the supervisory checking operation, the ,timer-contact TM5 opens, at the station M, thus deenergizing the long-interval timer TK, so that, in say, 0.1 second thereafter, or substantially instantaneously, the long-interval timercontacts TK50 and TK50 will both reset, that is, will both return to their normal open positions. The opening of the timer-contact TK50 Will interpose a second break in the grid-to-cathode circuit of the transmitter 6, so that the subsequent reclosure of the short-interval timer-contact TM4 will not restart the carrier-current transmitter when the short-interval timer TM resets, about a second later. The opening of the timer-contact TKil' makes it possible for the parallelconnected timer-contact 'I'MO to subsequently deenergize the short-interval timer TM.

At the end of the B-second interval after the initiation of the above-described supervisory check, the timer-contact TMO opens, deenergizing the short-interval timer TM at the station M. Within a very short-interval, such as 0.1 second, thereafter, the timer-contact TM recloses, restarting the 50-minute timerv TK. At the same time, the timer-contact TM4' reopens, silencing the check-back bell 2, and the timercontact TM4 recloses, making it possible for the long-interval timer-contact TK50 to subsequently restart the carrier-current transmission at said station M, at the end of another 50-minute interv val.

At the end of the 7-second interval after the iirst initiation of carrier-current transmission, sustained for at least three seconds, at the rst station, M, the timer TN at the second station, N, iinally opens its contact TNT, thus deenergizing said timer TN. Within a short time, such as 0.1 second, thereafter, the timer-contacts TN3 and TNS' reset, or reopen, the former stopping carrier-current transmission at said station N, and the latter removing the holding-circuit from the receiver relay RN, thus permitting said receiver relay to become deenergized simultanously with the cessation of carrier-'current transmission. The receiver-relay contact RN! reopens, thus preventing the timer TN from becoming reenergized when the timer-contact TNI returns to its normal closed position. At the same time, the receiver-relay contact RN2 opens, silencing the bell 4 at said station N. Upon the interruption of carrier-current transmission at the station N, the receiver relay RM is also de-energized at the station M, thus silencing the bell 3.

If the carrier-current equipment is operating properly, the bells 3 and 4 will both begin ringing instantaneously at the beginning of each check-back test, and they will remain ringing for approximately 7.1 seconds, while the bell 2 will remain silent for 4 seconds and will then ring for approximately 2.1 seconds, and the bell l will not ring at all. If the carrier-current transmitter 6 at the first station, M, is not operating properly, the bells 2, 3 and 4 will all remain silent, and the alarm-bell I Will begin ringing at the end of the i-second interval, and will continue thereafter to ring until the station-attendant shuts it off. If the carrier-current receiver 1 at the rst station, M, is incapable of receiving carrier-current transmitted from its own station M, the bells 2 and 3 will not ring at all, at this station, but the bell 4 will ring at the other station, N, and the alarm-bell l will start ringing, as before, at the end of thel 4-second interval. If the receiver 1 at the station M. can receive from its own transmitter 6, but not from the transmitter 8 at the other station N, either through a fault of the receiver 1, or from a failure of the carrier-current equipment at the other station, N, the bell 3 will ring for 3 seconds, instead of 7.1 seconds, the bell 2, will not ring at all, and the alarm-bell I will ring as previously described.

The foregoing operations thus aiord a complete check-up on the carrier-current equipment at both ends of the transmission-line 5, as well as a check-up on thefline. itself, so that the attendant at; the station M will hear the alarm-bell Iv if anything is wrong with the carrier-current equipment at either end of the line.

The 3-second time-interval which is provided by theA timer 'IN at the second station, N, makes it necessary for the carrier-current to` be continuously transmitted, from the rst station, M, for 3 seconds, before the check-back operation is assured, thus making it possible to utilize the carrier-current equipment in any manner which does not result in uninterrupted carrier-current transmission, from both ends combined, for an uninterrupted time as long as 3 seconds, without instigating a check-back supervisory initiation of carrier-current transmission at the second station, N. Thus, normal fault-responsive relaying operations will always be completed in much less than the 3second interval which is provided.

Furthermore,v the 3-second interval also makes it possible to signal. from one end of the transmission line 5, to the other, by means of the respective push buttons PBM and PBN. These push buttons are utilized to connect the respective oscillator-grids Il to their respective cathodes I9, so as to start carrier-current transmission at the corresponding end of the transmission-line 5.` Each time carrier-current transmission is started by pressing either push button PBM or PBN, the bells 3 and 4 are rung, in both stations, and these bells may be sounded, in codes of two or more rings, to call the attention of the operator at the other station, either as a telephone-bell to ask him to answer the telephone, or for any other purpose, provided that the callcode does not involve a continuous depression of either push button PBM or PBN for as long as 3 seconds at any one time.

While I have just described my invention in connection with a complete automatic equipment, which automatically instigates a rechecking supervisory operation at the end of each 5G-minute time-interval, or any other interval as controlled by the long-interval timer TK, it will be obvious that I am not limited to this complete equipment, in each case.

Thus, as indicated in Fig. 4, the long-interval timer TK can be dispensed with entirely, and its contact TKS, which initiates the supervisory carrier-current transmission at the-V station M, can be replaced by a push-button control. In Fig. 4, a push button PB is utilized to energize an auxiliary relay B having two instantaneous make contacts BI and B2. The contact Bi initiates carrier-current transmission, at its own station M', preferably, though not necessarily, in series with a timer-contact T4 corresponding to the timer-contact TM4 of Figs. 1 and 2, while the contact B2 energizes the timer T which corresponds to the timer 'IM of Figs. l and 2. The timer T has only the normally closed contact T4, which opens in 4 seconds, and the normally open contact T4' which closes in 4 seconds, the latter contact corresponding to the contact TM4' of Figs. 1 and 2. The receiver relay RM and the auxiliary relay A are connected and utilized, in Fig. 4, in the same manner as in the preferred or complete form of embodiment of my invention, as shown in Figs. 1 and 2.

In operation, in order to obtain a supervisory carrier-current check-back, with the equipment at the station M', as shown in Fig.v 4, and with the same equipment, at the other station, N, as is shown in Figs. 1 and 3, the push button PB is depressed at the station M', and is heid down .for at least 3 seconds, until the transmitter 8 Aupon `initiating la supervisory carrier-current at theA other station, N, begins to transmit. As a matter of practical operation,.the pushbutton PB may be held doWn untilthe check-back bell 2 rings, at the close of the 4-second interval, at Which. time the push lbutton PB should be released, so that it will be. releasedat least before the station N transmitter 8 stops transmitting at the end of the l7-second interval. The operation of the equipment shown inFig. 4 is other- Wise the same as Was described for Figs. 1, 2 and 3, except that the operator atthe station M must initiate each supervisory check-back operation, instead of having the long-interval timer .TK vto perform this service for him.

While I have illustrated my invention in two forms of embodiment which are. at present preferred by me, it Will be obvious that I am not limited to all of the details of apparatus or function which have been specifically described, and I desire my description and illustration to be understood in an illustrative sense, illustrating the general principles of my invention, rather than to be taken in a restrictive sense as limiting my invention to the precise details or processes shown. I desire, therefore, that the appended claims shall be accorded the broadest construction consistent With their language and the prior art.

I claim as my invention:

l. In a carrier-current system comprising a normally non-transmitting carrier-current transmitter and a carrier-current receiver at a rst station, and a normally non-transmitting carrier-current transmitter and a carrier-current receiver at a second station in communicable relation to said rst station, all of said carriercurrent apparatus being operative at the same carrier-current frequency,-the combination, with said system, of supervisory equipment comprising supervisory means for causing a supervisory carrier-current transmission at the first station, supervisory time-delay means responsive to a continued receipt of carrier for a rst predetermined time at the second station for thereupon initiating a supervisory carrier-current transmission at said second station and for maintaining said supervisory carrier-current transmission for a second predetermined time, supervisory time-delay means for facilitating the interruption of said supervisory carrier-current transmission at the rst station after a third predetermined time after the initiation of said supervisory carrier-current transmission, said third predetermined time being longer than said first predetermined time but shorter than the sum of said first and second predetermined times, and indicating means responsive, in any predetermined manner, to the various operations of said rst and second stations, respectively.

2. In a carrier-currentv system comprising a normally non-transmitting carrier-current transmitter and a carrier-current receiver at a iirst station, and a normally non-transmitting carrier-current transmitter and a carrier-current receiver Aat a second station in communicable relation to said first station, all of said carriercurrent apparatus being operative at the same carrier-current frequency, the combination, with said system, of supervisory equipment comprising. supervisory means for causing a supervisory carrier-current transmission at the rst station, supervisory time-delay means responsive to a continued receipt of carrier for a first predetermined time at the second station for theretransmission at lsaid second station andA for` maintaining Vsaid lsupervisory carrier-current trans- ,mission Vfora second predetermined time, supervisory timedelay/means forv facilitating the interruption i of said supervisory carrier-current transmission at, the yfirst station after a third predetermined time after the initiation of said supervisory carrier-current transmission, said third, predetermined time being longer than said `first predeterminedy time but shorter than the sum of saidv first and second predetermined times, and indicatingv means responsive to a nonreceipt o f a predetermined carrier current by the receiver at the first station after said interruption of saidl supervisory carrier-current transmission at said Ifirst station.

3. In combination, means for transmitting a signal .at a first station and for receiving it both at said'rst station and at a second station, means responsiveA to a continued receipt of said signal for a first predetermined time at said second station for thereupon sending acontinued signal, for a second predetermined time, to

lthe rst station and for receiving the last-mentioned signal at said first station, time-delay means for facilitating the interruption of said signal-transmission at said rst station after a third predetermined time after the initiation'of said signal-transmission, said-v third predetermined time being vlonger than said first predetermined time but shorter than the sum of said first and second predetermined times, and indicating means responsive to a non-receipt of a of said signal-transmission at said first station.

4. In combination, means for transmitting a signal at a first station and for receiving it at a second station, means responsive to a continued receipt of said signal for a iirst predetermined time at said second station for thereupon sending a continued signal, for a second predetermined time, to the iirst stationand for receiving thelast-mentioned signal at said first station, time-delay means for facilitating the interruption of said signal-transmission at said first station after a third predetermined time after the initiation of said signal-transmission, said third predetermined time being longer than said first predetermined time but shorter than the ,suml of said first and second predetermined times, and indicating means responsive to a nonreceipt of a signal at said first station after said interruption of said signal-transmission at said rst station.

5. In a carrier-current system comprising a normally non-transmitting carrier-current transmitter and a carrier-current receiver at a first station, and a normally non-transmitting carrier-current transmitter and a carrier-current receiver at a second station in communicable relation to said first station, all of said carrier-current apparatus being operative at the same carrier-current frequency, the combination, with said system, of supervisory equipment comprising supervisory means for causing a supervisory carrier-current transmission at the rst station, supervisory time-delay means responsive to a continued receipt of carrier for a rst predetermined time at the second station for thereupon initiating a supervisory carriercurrent transmission at said second station and for maintaining said supervisory carrier-current transmission for a second predetermined time, and supervisory time-delay means for providing vas signal at said first station after said interruption an indication of the receipt of a predetermined carrier current by the receiver at the irst-station after a third predetermined time after the initiation of the supervisory carrier-current transmission at said rst station, s aid third predetermined time being longer than said first predetermined time but shorter than the sum of said first and second predetermined times.

6. In combination, means for transmitting a signal at a rst station and for receiving it both at said first station and at a second station, means responsive to a continued receipt of said signal for a first predetermined time at ksaid second station for thereupon sending a continued signal, for a second predetermined time, to the first station and for receiving the lastmentioned signal at said rst station, and timedelay means for providing an indication of the receipt of a signal at the rst station after a third predetermined time. after the initiation of said signal-transmission at said rst station, said third predetermined time being longer than said first predetermined time but shorter than the sum of said rst and second predetermined station after a third predetermined time after the initiation of vsaid signal-transmission at said first station, said third predetermined time being longer than said first predetermined time .40 but shorter than the sum of said first kand second predetermined times.

8. The invention as defined in claim 2, in combination with supervisory time-delay means for providing an indication of the receipt of a 45 predetermined carrier current by the receiver at the rst .station after said third predetermined time.

9. The invention as dened in claim 3, Ain combination with time-delay means for providing an 50 indication of the receipt of a signal at the first station after said third predetermined time.

1.0. The invention as defined in claim 4, in

Acombination with time-delay means for providing an indication of the receipt of a signal at the rst station after said third predetermined time.

11. The .invention as dened in claim 2, in combination with quickly operating means for responding to the receipt of a predetermined carrier current by the receiver at at least one of said stations.

`12. VThe invention as defined in claim 3, in combination with quickly operating means for responding to the receipt of a signal at at least one of said stations.

13. The invention as defined in claim 4, in combination with quickly operating means for responding to the receipt of a signal at at least .one of said stations.

14. The invention as defined in claim 5, in combination with quickly operating means for responding .to .the receipt of a predetermined carrier current by the receiver at at least one of said stations.

15. The invention as dened in claim 6, in combination with quickly operating means for responding to the receipt of a Asignal at at least one of said stations.

16. The invention as dened in claim '7, in combination with quickly operating means for responding to the receipt of a signal at at least one of said stations.

17. The invention as defined in claim 1, in combination with means for automatically initiating the supervisory carrier-current transmission at the first station at long predetermined intervals.

18. The invention as defined in claim 4, in combination with means for automatically initiating the signal-transmission at the rst station at long predetermined intervals.

19. The invention as defined in claim 5, in combination with means for automatically initiating the supervisory carrier-current transmission at the rst station at long predetermined intervals.

20. 'I'he invention as defined in claim 7, in combination With means for automatically initiating the signal-transmission at the first station at long predetermined intervals.

SIMEON C. LEYLAND. 

